1. Field of the Invention
The invention relates to apparatus for recovering metallic silver from photographic processing solutions containing silver ions and, more particularly, to a compact, portable silver recovery unit having improved characteristics of assembly, plating efficiency, and disassembly.
2. Description of the Prior Art
In the development of x-ray film, movie film, and other types of film, an emulsion previously applied to the surface of the film is contacted by a solution of sodium thiosulfate (hypo). The hypo washes off unhardened silver-containing compound from the surface of the film after development; the silver-containing compound is dissolved and goes into solution. After a period of time, the hypo becomes saturated with silver and no longer is fit for use. Various proposals have been advanced for removing the silver from the hypo to recover the value of the silver and/or rejuvenate the hypo. Three basic methods of silver recovery have been proposed:
A. Metallic replacement. In the metallic replacement technique, silver-saturated hypo is brought into contact with a metal substance such as steel wool, zinc dust, or copper in a form having a large surface area. Silver is attracted out of solution by the metal substance. This is a fairly effective technique for recovering silver with only one basic drawback: The hypo is ruined and must be discarded. In addition to the obvious pollution problems, an entirely new batch of hypo is required and the savings from the efficient silver recovery are mitigated by the need to use more hypo. Moreover, it often is difficult to tell how much silver is in solution and, thus, unsaturated hypo solution may be processed and thrown away needlessly.
B. Chemical precipitation. In the chemical precipitation technique, various chemical compounds are added to silver-saturated hypo to precipitate the silver compound. The chemical precipitation technique suffers from the same drawbacks as does the metallic replacement technique in that the hypo is ruined and must be discarded. The chemical precipitation technique contains additional drawbacks in that the chemicals involved can be difficult to work with, and the efficiency of the process is comparatively low.
C. Electrolytic Precipitation. In the electrolytic technique, two electrodes, a cathode and an anode, are placed in a bath of hypo. Electric current is passed between the electrodes and silver is deposited on the cathode. The chief advantage of the electrolytic technique is that the hypo can be rejuvenated and reused to process additional film. When carried out properly, the efficiency of the electrolytic technique, although lower than the metallic replacement technique, is acceptable. Unfortunately, certain drawbacks of known electrolytic methods and apparatus exist which have made it difficult to employ the electrolytic technique to maximum advantage.
An important problem with the electrolytic technique deals with the cathode surface area presented to the hypo and with the current density required to efficiently remove silver from the hypo. If the current density and/or plating surface area is too low with respect to the amount of silver and other constituents present, the plating efficiency is not high enough and too much time will be required to remove silver from the hypo. On the other hand, if the current density is too high with respect to the amount of silver and other constituents present, silver sulfide will be formed at the cathode and plating of silver will be stopped. Additionally, the hypo will be made unfit for further use. Prior electrolytic methods and apparatus all have attempted to overcome the current density and plating surface area problem, although none has failed to achieve entirely satisfactory results.
In part, this is because it has been difficult, if not impossible, to properly move the hypo past the anodes and the cathodes such that the maximum amount of surface area of the cathodes is presented to the hypo without creating voids or eddy currents. Unless the flow of hypo across the cathode is substantially laminar, the current density applied to the hypo will be variable. In turn, areas of higher current density will initiate the formation of silver sulfide. If continued long enough, the initially formed silver sulfide will spread to other areas of the hypo and the hypo eventually will be ruined. Even if the hypo is not ruined, irregular plating activity can occur and the efficiency of the plating operation decreased.
A basic problem, then, with the electrolytic technique is to effectively bring the hypo into contact with a large cathode surface area so that a large amount of silver can be plated without bringing about the formation of silver sulfide. The problem has been approached fairly well in large installations. In these installations, large, stationary, plate-like anodes and cathodes are spaced a small distance apart and stirrers or paddles are moved slowly between adjacent plates. Installations such as the type described are very large and expensive, and have not been made small enough to be portable. Also, considerable effort is required to remove the cathode plates and strip them of plated silver. Although these large installations are suitable for processing substantial volumes of hypo on a more or less continuous basis, they are unsuited for processing smaller quantities of hypo on a demand basis such as occurs commonly in hospitals, small film processing laboratories, and so forth.
Although portable electrolytic units exist, they all suffer from the drawback of not being able to process the hypo as efficiently as desired. One known unit suspends an elongate cathode assembly vertically in a tank of hypo. The cathode assembly includes a shaft along which a plurality of circular discs are disposed equidistantly. The cathode assembly is rotated in use so that a "shearing" action takes place between the cathode discs and the hypo. Although this device functions well in theory to produce laminar flow across the face of the cathode discs, it does not work as well in practice. In part, this is because silver does not plate evenly onto the surface of the discs and turbulent flow eventually is created after a certain amount of silver has been plated onto the discs. Also, bubbles can be trapped on the underside of the discs and flow irregularities thereby created. Moreover, because the cathode assembly is suspended at only one point, excessive stress is applied to a support bearing and other drive shaft components by which the cathode assembly is suspended.
Other portable devices employ differently configured cathode assemblies, but problems with the cathode assemblies still remain. For example, cylindrical, stationary cathodes have been used in which hypo is moved past the cathode by means of a pump or an impeller. Although a large surface area is presented to the hypo, turbulent flow can result from the technique by which the hypo is pumped and it is difficult to strip plated silver from the cathode without scratching or deforming the cylindrical cathode. In short, all known portable silver recovery devices have had serious problems with respect to (a) properly moving the hypo and the cathode relative to each other for maximum plating efficiency and (b) removing the cathode and stripping plated silver quickly without damaging the cathode components.